1. Field of the Invention
The present invention relates to the interior cleaning of pipes, conduit and the like. More particularly, the present invention relates to removal of scale and other solid deposits from the inside surfaces of pipes, conduits and the like by means of a high velocity gas stream.
2. Description of the Background
The interior walls or surfaces of pipes used in commercial installations, such as chemical plants, refineries, power generating plants, etc., are frequently coated with or have deposits of solid materials which have been deposited on the walls from the fluids passing through the pipes or have been left on the walls during manufacture of the pipe. These solid deposits on the interior of piping walls can lead to serious problems. Such deposits can interfer with desired flow patterns in the piping. Additionally, since the deposits may periodically dislodge into the flowing fluid, they can cause damage to or interfere with downstream equipment or processes. For example, steam used to drive a steam turbine must be free from debris entrained in the steam. Debris such as scale, slag or the like which may become dislodged from steam lines used to feed steam turbines can be accelerated to high velocities by the steam and at such velocities impinge on the turbube blade surfaces with sufficient force to damage the blades or other related equipment. Likewise, steam lines which supply steam to finely machined valves used for reducing or relieving excess pressure must be free of scale and debris which can cause serious damage to the valve internals if it becomes entrained in the flowing steam. Steam lines which convey steam to reactors or other process vessels must also be free of wall deposits and comtaminates which can become entrained in the steam and possibly interfere with the process reactions.
In a typical prior art method of cleaning steam lines, it is customary to go through a cycle of heating, cooling and steam blowing. Thus, pressure is built up in the boiler and then released through the steam lines to be cleaned. The lines are then allowed to cool while steam pressure is built up again in the boiler. The cycle of heating, cooling and blowing is repeated until the steam emerging from the blow down piping is observed to be clean. In determining if the steam is clean, it is normal practice to blow the line with steam until a piece of metal, called a target, supported across the exhausting flow of steam shows no indication of debris impingement on its exposed surfaces. Typically, the number of such targets used may range from five to several hundred before a clean target is obtained indicating that the lines are free of loose debris or scale. This prior art method of steam cleaning piping is described in a publication of the General Electric Company entitled "Instructions-Cleaning of Main Steam Piping and Provisions for Hydrostatic Testing of Reheater," incorporated herein by reference.
It is also known, as taught in U.S. Pat. No. 3,084,076 to use chemical cleaning in which certain solvents or additives are added to the steam to dissolve scale, residue and other deposits from the lines being cleaned.
In another prior art method for steam line blowing or cleaning, air and water are simultaneously added to the flowing steam in the pipe to be cleaned to generate an annular mist of condensate droplets to penetrate laminar flow conditions at the pipe wall and thereby help to loosen debris from the pipe wall. In this prior art technique, it has been found desirable to effect complete condensation of the exhausting steam to reduce system pressure.
While the prior art methods discussed above are partially effective for cleaning of steam lines, they suffer from several drawbacks such as (1) excessive noise from the exhausting steam, (2) excessive requirements for steam in the cleaning, (3) disposal problems related to chemical usage or massive quantities of condensing water, (4) requirements for numerous exhaust blows to either build steam pressure or target changeout, (5) uncertainty related to the absolute degree of cleanliness obtained, (6) the time-consuming and expensive requirement to anchor the piping being cleaned to withstand reaction forces.